It is well known to the art that condensation of a phenol and an aldehyde provides materials curable to thermoset phenolic resins. Base-catalyzed condensation employing at least about a stoichiometric amount of aldehyde provides condensates known as resoles whereas acid catalysts and a deficiency of aldehyde provides novolacs. Characteristic of both liquid and solid resoles is their heat-curability to fully cross-linked, infusible products without the need for an added cross-linking agent. From this standpoint, resoles are more descriptively referred to as One-Step phenolic resins in contract to novolacs or Two-Step resins which do require the addition of a cross-linking agent for the curing process. The reactivity of resoles and ability to self-condense to higher molecular weight resins is attributable to the presence of hydroxymethyl groups which become bonded to the aromatic phenolic nucleus during the base-catalyzed condensation. From the standpoint of commercial application, the most significant resoles are those derived from phenol itself and formaldehyde.
Curing of resoles to higher molecular weight, cross-linked thermoset resins proceeds with generation of heat and is accelerated by acid materials. In the presence of strongly acidic accelerators of the exothermic reaction and a source of blowing action, liquid resoles cure rapidly to cellular phenolic resins. Although not essential to the formation of phenolic foam, it is usual practice to include a surface active agent as additional component of the phenolic foam formulation. Among the surface active agents described in the literature as suitable stabilizers of phenolic foam are the siloxane-oxyalkylene copolymers described in U.S. Pat. No. 3,271,331. These copolymers are comprised of: (1) a siloxane portion in which silicon atoms have a monovalent hydrocarbon group bonded thereto; and (2) an oxyalkylene portion consisting essentially of at least one oxyalkylene group. In the oxyalkylene portion of such stabilizers, an oxygen atom is bonded to carbon of a bivalent hydrocarbon group, the second valence of the bivalent group being bonded to silicon of the siloxane portion of the copolymer. Among the variety of monovalent hydrocarbon groups which can be bonded to silicon of the siloxane portion of such copolymers are alkyl groups such as methyl, aryl groups such as phenyl and aralkyl groups such as benzyl and phenylethyl.
Although phenolic foams are used to embed floral arrangements and for general packaging purposes, they have not found the widespread industrial application enjoyed by cellular polyurethanes. As compared with cellular polyurethanes, phenolic foams possess better inherent resistance to burn with an open flame, and emit very low levels of smoke on heating. Consequently, greater attention is being focused on phenolic foam technology so as to develop practical products having more widespread end-use applications. Among the undesirable properties of phenolic foams generally, is their ability to undergo a flameless combustion when heated to their decomposition temperature. This phenomenon is commonly referred to as punking and may be likened to the glowing of charcoal briquettes. Although temperatures well in excess of 500.degree. C. are necessary to initiate punking of phenolic foam, once started the hazardous punking or afterglow state is self-sustaining and sometimes foam temperatures as high as 1700.degree. C. are reached. The punking properties of phenolic foam is recognized in the prior art and a number of methods for achieving punking resistance have been reported. For example, production of non punking foams has been previously reported in U.S. Pat. No. 3,298,973 by the employment of a particular catalyst mixture of at least two acidic agents. The catalyst is a solid mixture of boric acid, or its anhydride, and an organic hydroxy acid in which the hydroxy group is on a carbon atom not more than one carbon atom removed from a carboxyl group. Punking of phenolic foams is also inhibited by the incorporation of certain organophosphorus compounds described in U.S. Pat. No. 3,673,130. Unfortunately, these techniques have a number of drawbacks. One is that they generally require relatively high quantities of the non punking component. Further, the aforementioned catalyst mixture based on boric acid or its anhydride is a solid which hampers handling and foam processability in that it is relatively difficult to disperse the solid catalyst into the liquid resole quickly and uniformly so as to achieve an even foam rise. Among the drawbacks associated with organophosphorus compounds as non punking additives is that they are, in general, expensive and inefficient from the standpoint that relatively high concentrations thereof (such as, for example, 5 to 20 parts, and usually at least 15 parts per 100 parts of resole) are required to impart punking resistance.
It is desirable, therefore, and is a primary objective of this invention to provide an improved method for the formation of phenolic foam having improved punking resistance.
Another object is to provide non punking phenolic foam by the use of a material which is both normally liquid and effective at relatively low concentrations.
Various other objects and advantages of this invention will become apparent to those skilled in the art from the accompanying description and disclosure.